load attribute_accessors for ActiveSupport 4

When using ActiveSupport 4, we must explicitly load attribute_accessors
in order to use the mattr_accessor function in ExceptionNotifier.

Tested with ActiveSupport 4.0.0.

(Note that we only hit this bug when using ExceptionNotifier outside of
Bundler, since Gemfile.lock is presently locking activesupport to 3.2. I
assume that activesupport in exception_notification's Gemfile.lock will
be updated at some point in the future, at which point this will become
relevant to Bundler users as well.)
Merge pull request #185 from ktdreyer/as4

load attribute_accessors for ActiveSupport 4